The present invention relates generally to Code Division Multiple Access (CDMA) systems and, more particularly, to reducing multipath interference in a received CDMA signal.
Different types of wireless communication systems have adopted various schemes for supporting as many simultaneous users as possible. Code Division Multiple Access (CDMA) is one such scheme. CDMA is a technique employed in spread spectrum communications systems that allows multiple users to simultaneously share the same frequency. In CDMA systems, a wideband spreading signal is used to convert a narrowband data signal into a wideband signal for transmission. Direct sequence spread spectrum systems use a pseudo noise (PN) sequence to spread the data signal into a wideband signal.
Modulation of the PN sequence by the data sequence may be achieved by, for example, applying the data sequence and the PN sequence to a product modulator or multiplier. Multiplication of two signals produces a resultant signal whose frequency spectrum is equal to the convolution of the frequency spectrum of the two signals being multiplied. Thus, multiplying the wideband PN sequence signal with the relatively narrowband data signal produces a wideband signal whose spectrum is nearly equal to the spectrum of the PN signal.
CDMA permits multiple users to simultaneously use the same frequency by assigning to each user a different PN code selected from a set of orthogonal codes. Transmissions to and from individual users are spread using their assigned PN codes. Thus, an individual user's signal may be recovered using his or her assigned spreading code from the combined CDMA signal. With orthogonal spreading codes, the cross-correlation between different signals spread using different codes is nominally zero. Thus, correlating a received CDMA signal, which contains signals for all active users sharing that frequency, with a given user's PN code results in recovery of the narrowband data signal spread with that user's PN code. Data signals intended for the other active users are not de-spread by correlation, and appear as wideband noise.
In CDMA systems using orthogonal codes and relatively long spreading code sequences with respect to transmitted data symbol timing, receivers can exploit these favorable code cross-correlation properties to increase receiver performance. Improvements in performance may be had even in the presence of unfavorable reception conditions, such as in multipath environments. Multipath reception commonly occurs in mobile terminals where numerous and changing obstructions intervene between the transmitter and the mobile terminals. In these cases, the transmitted CDMA signal travels to a given mobile terminal through a number of different propagation paths, with each path having different path characteristics, such as path length, phase, and attenuation.
RAKE receivers are frequently used in CDMA systems, and can exploit multipath reception in many circumstances to improve reception performance. RAKE receivers accomplish this by separately processing a selected number of the multipath versions of the CDMA signal received by the mobile receiver, and then coherently combining the data signals recovered from the selected multipath signals to form an overall RAKE receiver output signal with an improved signal-to-noise plus interference ratio (SNIR). RAKE receivers conventionally comprise some number of RAKE “fingers.” Each RAKE finger is adapted to correlate the received CDMA signal with a commonly assigned PN code. Each RAKE finger is time-adjusted to align it with a different one of the multipath versions of the CDMA signal received by the mobile receiver.
The time-alignment essentially “shifts” each RAKE finger in correspondence with a relative propagation path delay for one of the selected propagation paths. Propagation paths are generally selected based on signal strength, and a typical RAKE receiver includes only a limited number of RAKE fingers, which in operation are aligned with the most significant multipath signals. The individual RAKE finger outputs are typically weighted based on the propagation path characteristics of the corresponding multipath channels, and then coherently combined to form the overall RAKE receiver output signal. Conventional RAKE receiver techniques generally work best in environments where the total number of simultaneous users occupying the same frequency is not too great. As the number of simultaneous users increases, the amount of interference arising from the multipath signals influencing each other increases.